Sure, one can find out how to build a bomb on the net. However, the challenge is to build a reliable device that can be carried(challenge to reliability) and deposited without killing the terrorist. Not that easy for a soccer player and a boxer. In fact, probably a full time job for an engineering student with electronics experience. And, who fabricated the circuit board? Who paid for the Mercedes/Porsche? Who paid the rent/educational expenses?
Where did the older brother get the money to live in Russia for a few months? Why did Russian find his actions odd enough to warn us he might be a terrorist?
Don’t forget the 6-month trip to Russia. What was he doing there and who paid for that? And why did he go there if, as claimed, he was a refugee? Aren’t refugees supposed to be desperately fleeing from persecution? Not happily commuting back and forth?